This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. xc2xa7119 arising from an application entitled, METHOD FOR CONTROLLING FORWARD POWER UTILIZING AN ERASURE INDICATOR BIT INA CDMA SYSTEM, earlier filed in the Korean Industrial Property Office on Jul. 29, 1998, and there duly assigned Serial No. 1998-30483.
The present invention relates to a method used in a mobile communication system for controlling base station forward transmission power utilizing erasure indicator bit (EIB) message, and in particularly to a forward power control method for improving call quality and subscriber capacity of the system using EIB message to adjust the gain of the base station forward transmission power and the rate of the reducing the digital gain of its forward transmission power based on a newly predetermined power control parameter messages stored in the base station.
As it is well known, a near-far problem arises in the channel quality between a base station and a faraway mobile station. In view of this, a forward power control has been studied to overcome the near-far problem. The forward power control is performed when a frame error at a base station exceeds a predetermined threshold level, in which case the transmission power of the base station to the mobile station is adjusted. The power control on the forward link increases or decreases the transmit power level of base station so that all channels can maintain good call quality when fading or interference occurs, especially in the boundary of a cell where the noise interference is high.
The power control is used to adjust the transmit power of the base station for each traffic channel. When the mobile station is affected by the signals traveling through propagation paths or other multi-path characteristic interference, the transmission power control is performed such that the received power levels of signals transmitted from the mobile stations and receive by the base station, or the SIR (Signal-to-interference power ratio) associated with the received power levels are kept constant at the base station.
Without the power control, the distance of the mobile station located in a neighboring area between base stations is so faraway from the corresponding base station that the transmit power from the base station is weakened. Consequently, a call set-up rate is low and even if the call set-up is successful, the voice quality is poor due to the severe interference in the neighboring base stations.
It will be appreciated by those skilled in the relevant art based on the detailed description provided herein that in a typical forward power control known in the art include a mobile station unit for measuring the quality of a communication signal received by the mobile station from a base station and a unit for producing a base station transmission power control data/forward power control signal, and the base station include a unit for measuring a receiving the forward power control signal, a unit for calculating a power level command based on the received forward power control signal based on control parameters stored in the base station, and a unit for producing a mobile station transmission power control data specifying a mobile station transmission power in accordance with the received.
In power control technique, the mobile station measures the frame error rate (FER) of the forward link by 20 milliseconds (ms) and transmits a forward power control signal to the base station. The base station compares the forward power control signal to a predetermined FER threshold value, and based on such comparison changes the transmission power level by increasing/decreasing the power level allocated to the traffic channel at an increment/decrement rate.
They are different types of forward control signal that is used depending on the system employed. There are power measurement report message (PMRM) used as a forward power control signal in the digital cellular system and erasure indicate bit (EIB) used in the personal communication system (PCS). The PCS makes it possible to report the status of the forward link every frame by adding a bit of EIB to the front of the reverse traffic channel. EIB indicates whether the received frame from the base station to mobile station contains an erasure. For an example, an EIB binary value of xe2x80x9c0xe2x80x9d can indicate a properly received frame, while a value of xe2x80x9c1xe2x80x9d can indicates an erased frame or bad frames. That is, in the event that mobile station is unable to successfully decode in response to forward frames, mobile station generates EIB messages having a value of xe2x80x9c1xe2x80x9d for each erased frame, and transmits these messages back to base station over the reverse link signal.
Base station processes the EIB and determines the gain of its forward link signal based on the EIB messages it receives on the reverse link signal from the mobile station. Base station transmits the forward link signal at a decreasing transmission gain or at an increased gain whenever an EIB message indicating an error is received, and increases or decreases the forward transmission power level transmitted by base station in the subsequent forward frame. By producing the power control signal in such away, it is possible to keep track of the change in the traffic channel more quickly than employing the PMRM method used in the PCS system. The base station receives these forward power control signal from the mobile station via reverse link and extracts the transmission power control bit, and determines its transmission power in accordance with the transmission power control bit.
As described in the above, the forward power control employing the EIB, the base station forward transmission power is depended on power control parameters messages stored in the base station, that is characteristic of the distance between the base station and the mobile station so that the power of the transmitted radio signals as received by the mobile station is not interfering with other cells. The related power control parameters are as follows: xe2x80x98Step_til_downxe2x80x99 for decreasing the digital gain by one per frame if the current traffic channel gain is greater than or equal to a xe2x80x98slow_down_level,xe2x80x99 and for decreasing the digital gain by one per every two frames if the current traffic channel gain is less than the xe2x80x98slow_down_level,xe2x80x99 xe2x80x98Down_deltaxe2x80x99 for decreasing the digital gain, xe2x80x98Nominal_gainxe2x80x99 refers to an initial gain of the primary traffic channel when a call is established between the mobile station and the base station, xe2x80x98Max_tx_gainxe2x80x99 refers to a maximum gain of traffic channel, xe2x80x98min_tx_gainxe2x80x99 refers to a minimum gain of traffic channel, xe2x80x98Big_up_deltaxe2x80x99 for increasing the digital gain, xe2x80x98small_up_deltaxe2x80x99 for increasing the digital gain that is smaller than the digital gain for the xe2x80x98Big_up_delta,xe2x80x99 and, xe2x80x98Slow_down_levelxe2x80x99 refers to a value at the level that is bottom one-third down between levels of the max_tx_gain and min_tx_gain.
In operation of the forward power control using the above power control parameters, FIG. 1 illustrates the digital gain increment of the traffic channel in which the PCS increases the digital gain of the traffic channel in case where the value of EIB is 1. Whenever a frame generating EIB message of xe2x80x981xe2x80x99 is received, it adjusts the gain value to either the xe2x80x98small_up_deltaxe2x80x99 and the xe2x80x98big_up_deltaxe2x80x99 depending on the sequence of received EIB messages. For an example, when twelve frames having a string of EIB messages of xe2x80x981xe2x80x99 or xe2x80x980xe2x80x99 are received as illustrated in FIG. 1 and FIG. 2, the gain is increased as high as xe2x80x985xe2x80x99 initially when EIB message of the first frame corresponds to xe2x80x981,xe2x80x99 and then the subsequently received EIB message of xe2x80x981xe2x80x99 increases the digital gain to as high as xe2x80x982xe2x80x99 three times. That is, the gain is increased with a specific pattern, xe2x80x985, 2, 2, 2xe2x80x99 in sequence. After that, if subsequent repetitive receptions of the EIB message of xe2x80x9c1xe2x80x9d is received, the value of the subsequent digital gain is adjusted according to the pattern shown in FIG. 2.
Referring to FIG. 3 that is a view of digital gain decrement of the traffic channel according to a conventional method. Whenever the repetitive receptions of a string of EIB messages of xe2x80x980xe2x80x99 (zero) have been continuously received for each 60 frames, the traffic channel gain is decreased by xe2x80x981xe2x80x99 for each 60 frames in sequence if the traffic channel is greater than or equal to xe2x80x98slow_down_level.xe2x80x99 However, if the traffic channel is less than xe2x80x98slow_down_level,xe2x80x99 the traffic channel gain is decreased by xe2x80x981xe2x80x99 for each 120 frames when a string of the EIB messages of xe2x80x980xe2x80x99 (zero) have been repetitively received.
The forward power control based on a predetermined control parameters of forward transmission power of the base station and a string of the EIB messages has help to improve the call quality. In the conventional method of establishing the parameters of the forward power control, the parameters are set only to address to improve the call quality, and not pertained to the subscriber capacity. Thus, there is need for establish the parameters to improve both the subscriber capacity as well as to the call quality. In order to achieve such improvements, the present invention provides a set of control parameters contrary to technologies known in the prior art, which produces an expected improvement in both the call quality and the subscriber capacity. This is done by setting the control parameter for the base station forward transmission power in an optimal way to adjust the digital gain of the base station forward link signal at a specific gain reduction rate, which results in an improvement in the call quality and a higher subscriber capacity compare to the current scheme of the forward power control known in the CDMA system.
The present invention is intended to provide a method for controlling forward transmission power utilizing erasure indicator bit (EIB) in accordance with a newly set control parameters to yield better call quality and higher subscriber capacity, by re-adjusting the control parameter in a conventional CDMA system.
A preferable embodiment of forward power control method using 13K erasure indicator bit (EIB) in a CDMA system according to the present invention improves the call quality and subscriber capacity by adjusting power control parameter messages which include a nominal_gain, a max_tx_gin, a min_tx_gain, and a periodic_frame_count, wherein the nominal_gain represents an initial gain of the primary traffic channel, the max_tx_gain represents a maximum gain of the traffic channel, min_tx_gain represents a minimum gain of the traffic channel, and a periodic_frame_count represents the rate of decreasing the digital gain by xe2x80x9c1.xe2x80x9d
According to the preferable embodiment of the present invention, it is preferable that the nominal_gain satisfies the following condition:                     E        b                    N        o              =                            E          c                          I          o                    +              P        .        G            +              10        ⁢                  xe2x80x83                ⁢                  log          ⁡                      [                                                            (                                      G                    T                                    )                                2                                                              (                                      G                    P                                    )                                2                                      ]                                ,
where       E    b        N    o  
is signal strength of traffic channel,       E    c        I    o  
is pilot signal strength, P.G is power gain, GP is pilot signal gain, and GT is the nominal gain, and the nominal_gain is adjusted so that a mobile station can have signal strength of traffic channel about 5 dB which is required to satisfy the minimum call quality or more, but keeping the pilot signal strength at about xe2x88x9214 dB.
According to the preferable embodiment of the present invention, it is preferable that the variable value of the nominal_gain is set to 100 compare to 70 as used in the prior art.
According to the preferable embodiment of the present invention, it is preferable that the reduction rate is adjusted to decrease the digital gain by xe2x80x9c1xe2x80x9d whenever the number of properly received frames are received for 20 counts if the level of a traffic channel gain (tc_gain) is greater than or equal to a slow-down level, wherein the slow-down level indicates a gain level being at the bottom one-third between the max_tx_gain and min_tx_gain, and to decrease the digital gain by xe2x80x9c1xe2x80x9d whenever the number of properly received frames are received as much as 40 counts if the level of traffic channel gain (tc_gain) is less than the slow-down level.
According to the preferable embodiment of the present invention, it is preferable that the value for maximum gain of the traffic channel is set to 127, wherein the value of 90 was used for the maximum gain in a known art.
According to the preferable embodiment of the present invention, it is preferable that the value for a minimum gain of the traffic channel is set to 35, wherein the value of 40 was used for the minimum gain in a known art.
According to the preferable embodiment of the present invention, it is preferable that the value of a maximum gain of the traffic channel is set to 127 and the minimum gain of the traffic channel is set to 35.